I'm using Team Foundation Server 2010 with Visual Studio 2010.
Whenever I modify a file outside of Visual Studio, TFS doesn't seem to detect the change done to the file, and thus doesn't offer me the option to check-in the file after it has been modified.
How can this be solved?
If you have a network connection to your server while you're working outside of Visual Studio, it's probably best to go ahead and check the file out before editing it, either using the tf command line client, or using the Windows Explorer shell integration that's available in the TFS Power Tools release. (Plus an increasing number of other tools have TFS integration that makes this automatic, but if you're just using notepad, this still needs to be a manual step.)
Of course, there are many times when you're working and you don't have a network connection available that allows you to check out the files.
If you know what files you've modified, you can just check them out from within Visual Studio, then you'll be able to check them back in.
If you don't know what files you've edited, you can detect the changes by running the tfpt online command (also part of the Power Tools release). This will locate the files that have been modified locally and check these files out from the server.